Dr Bernard Rosof, of the Long Island Jewish Health System, and colleagues work on the Physician Consortium for Performance Improvement.

It records why therapies are not used as 'medical reason', 'patient reason', 'system reason' or 'reason not given', and reports exception rates.

Examining these highlights information on clinical conditions and patient preferences, the researchers said. 'We are learning how to use these data to improve patient care,' they added.

Professor Helen Lester, a GP and DoH quality framework adviser, said: 'A fine-grained analysis of clinical practice, particularly in exception reporting, will add to our understanding of how to use pay for performance incentives to improve quality.'